Buffered read fills buffer if final value is missing

[kentcb]

The following test fails (read is 100, not 2):

[Fact]
public async void kbcsv_repro()
{
    var csv = @"Col1,Col2,Col3
val1,val2,val3
val1,val2,";

    using (var reader = CsvReader.FromCsvString(csv))
    {
        await reader.ReadHeaderRecordAsync();
        var buffer = new DataRecord[100];
        var read = await reader.ReadDataRecordsAsync(buffer, 0, buffer.Length);

        Assert.Equal(2, read);
    }
}

Including a final value or a new line is enough to make the test pass, but neither should be required.